The present invention relates to a developing device for use in an image forming apparatus such as a copying apparatus, etc.
Conventionally, developing devices are widely used. In the known developing devices, a developing sleeve is disposed adjacent to an electrostatic latent image support member so as to be driven to rotate and a plurality of magnets are securely provided in the developing sleeve and along an outer peripheral surface of the developing sleeve such that magnetic poles of the magnets extend in an axial direction of the developing sleeve. Thus, developer supplied to the surface of the developing sleeve is magnetically held on the surface of the developing sleeve by the magnets so as to form a magnetic brush on the surface of the developing sleeve. Then, during rotation of the developing sleeve, the above-described magnetic brush rubs against the surface of the electrostatic latent image support member such that the electrostatic latent image is developed into a visible image. In the known developing devices of the above-described type, in order to regulate the amount of the developer transported to a developing region where the developing sleeve and the electrostatic latent image support member confront each other, a bristle height regulating plate is usually provided so as to confront the developing sleeve.
Meanwhile, a prior art developing device employing a two-component developer composed of toner and carrier has a drawback in that since the toner and the carrier are stirred to be mixed with the toner being electrically charged through friction between the toner and the carrier, a stirring member for stirring the developer is required in addition to the developing sleeve.
In response to a recent trend in copying apparatuses towards compactness, there is a high demand for a compact developing device.
Thus, in order to make the developing device compact, the present inventors considered that the developer is stirred on the outer peripheral surface of the developing sleeve. To this end, a method was employed in which the advance of the developer transported on the outer peripheral surface of the developing sleeve is blocked by the above-described bristle height regulating plate such that convection of the developer is caused.
However, a distal end portion of the bristle height regulating member usually confronts a dense portion of the magnetic brush, which is disposed between opposite magnetic poles each having a large magnetic force so as to be inclined towards the outer peripheral surface of the developing sleeve. Therefore, the developer cut off by the bristle height regulating member is pushed by the developer subsequently transported so as to rise along the bristle height regulating member. However, since an attractive force of the strong magnetic poles is larger than the rising force of the developer, the developer cannot rise smoothly along the bristle height regulating member, so that lumps of the developer are formed in front of the bristle height regulating member and thus, in the worst case, clogging of the developer possibly takes place, thereby resulting in a state in which it is impossible to perform development.
Meanwhile, even if the developer rises along the bristle height regulating member, the amount of the developer which has risen along the bristle height regulating member is small and falls in an unmixed state on the surface of the developing sleeve intermittently. Hence, an inconvenience is incurred in which the amount of the developer transported to the developing region becomes nonuniform with the result that the nonuniform density appears on the copied image, thus resulting in an improper image.
Furthermore, since the bristle height regulating member is brought into contact with the magnetic brush in a dense state, resistance between the bristle height regulating member and the magnetic brush is great. As a result, a disadvantage of a large load being applied to a driving source for driving the developing sleeve transporting the developer is present. And, since the amount of the developer transported to the developing region varies greatly according to a slight error in adjustment of a gap between the bristle height regulating member and the developing sleeve, it is troublesome to adjust the gap.